The present invention relates to the suppression of a disturbance signal component of a communication signal, and more particularly to systems and methods for suppressing a periodic disturbance signal component having a known, or determinable, fundamental frequency, and its harmonic frequencies.
Many digital wireless systems in use today utilize a time slotted access system. An information signal (e.g., speech, data, video) is segmented, compressed, packetized and transmitted in a pre-allocated time slot. Time slots can be allocated to different users, a scheme commonly referred to as Time Division Multiple Access (TDMA). TDMA communication systems, such as the Global System for Mobile communications (GSM) in Europe, the Digital-Advanced Mobile Phone System (D-AMPS) system in North America, or the Personal Digital Cellular (PDC) system in Japan, allow a single radio frequency channel to be shared between multiple remote terminals, thereby increasing the capacity of the communication system. Also, Code Division Multiple Access (CDMA) access techniques use a framing structure to gather and transmit information across an air interface.
Time slots may also be allocated between alternating uplink and downlink transmissions, a scheme commonly referred to as Time Division Duplex (TDD). In a TDD system, the transmitter is inactive for a period of time during each frame, which period is of sufficient duration to receive a signal burst. The transmitter compensates for the loss of transmission time caused by this inactive period by buffering the digitized communication signal in a memory and subsequently transmitting the buffered communication signal at a higher rate than the rate at which it was buffered during its allocated slot. The peak transmitter power is increased by the same factor to support the higher rate.
In a GSM system, the TDMA circuits are switched on and off at a frequency of approximately 217 Hz. Switching the TDMA circuits creates a disturbance component at this frequency, referred to herein as the fundamental frequency, and its harmonic frequencies. The disturbance component is coupled into the communication signal, and may interfere with the information signal component of the communication signal. When the information signal component represents speech input, the disturbance signal component, if not suppressed, can cause an audible buzz, sometimes referred to as a “bumblebee” noise in the communication signal.
Existing radiotelephone or cellular communication systems suppress the bumblebee noise using various analog noise suppression techniques. For example, bumblebee noise can be suppressed by electrically decoupling the radio circuits, or by using microphones adapted to minimize the noise. Also digital techniques, such as digital noise cancellors, can be used to suppress bumblebee noise. However, digital noise chancellors are adaptive in nature, i.e., they estimate the noise and do not make use of prior knowledge of disturbing frequencies. As such, these techniques require costly components and can be difficult to implement. These techniques can also require the use of non-optimal system settings such as, for example, compensating offsets in microphone gain.
Linear notch filters can also be used to filter disturbance signals at known frequencies. However, an analysis of a Fourier expansion of a disturbing periodic signal that creates a bumblebee noise illustrates that the rate of decay of the frequency components of the disturbance signal is inversely proportional to the frequency (e.g., 1/frequency). Consequently, it is not effective to filter only the first few frequency components of the disturbing signal, because there are approximately fifteen frequency components having magnitudes large enough that they must be suppressed in the audible frequency band below 4 KHz. The computational complexity of implementing fifteen notch filters renders this option undesirable.
Accordingly, there is a need in the art for systems and methods for suppressing periodic disturbance components from communication signals.